MEGA65: VIC-IV (alt-)palette related issues

[lgblgblgb]

Mirage_BD on Discord has some very useful observations (thanks!) in terms of bugs in Xemu's VIC-IV emulation. This issue stands here as a reminder for me not to forget these and also as a place to discuss further this (these) issues.

The thread on Discord: https://discord.com/channels/719326990221574164/781481205639020554/1249020115085955123

Test program: megajpg.d81.zip

The highlights (so far):

The good: MEGAJPG seems to work (compared to 2023/July/2 state) basically

The bad: ... however the file selector has a strange red background which was not the case before:

His suggestion:

if bold and reverse both need to be set, shouldn't SXA_ATTR_ALTPALETTE(cw) in vic4.h be this:
#define SXA_ATTR_ALTPALETTE(cw) (((cw) & 0x0060) == 0x0060) instead of just ((cw) & 0x0060)
[...]
Just had another look at it, and the other thing you'll have to change is line 1474 of vic4.c:
const Uint32 *palette_now = ((REG_VICIII_ATTRIBS) && SXA_ATTR_ALTPALETTE(color_data)) ? altpalette : used_palette;
The change being the extra check for REG_VICIII_ATTRIBS... The alternate palette only has to be selected if bold+reverse and
vic3-attribs are enabled. That with the fix for the #define fixes all the issues in my programs.

And the ugly: bad border colour?

Unknown if there is anything related to the previous topic here, or something independent as a bug. Xemu currently does this, when it needs to render border colour: palette[REG_BORDER_COLOR]. Maybe this should be done?

used_palette[REG_BORDER_COLOR]

Images got from Discord and were taken by Mirage_BD.